1. Field of the Invention
The invention is directed to mine roof support fixtures and particularly to the placement of the grout anchor which holds the end of a cable in a borehole where anchorage is established through the use of a resinous or cementeous grout.
2. Description of the Prior Art
It is known in the creation of underground passages, that geologic forces are released which cause strains in the earth surrounding the mine passage and that strain is reflected by movements of geological material making up the roof of the passage. Means for resisting the forces to re-establish balance are support fixtures that retard movement of the geologic material, not only the roof, but around the passage. Such fixtures can take several different forms. One form is to use cable anchored devices which extend from the surface of the opening in the boreholes to the base of the borehole and are anchored in place through the use of resinous or cementeous grouts. Grouts completely fill the annulus of the hole between the cable and the surrounding geologic mass. Grout anchors are normally of some considerable length, say 2 to 5 feet in length, and may fill the entire annulus in the borehole. It is known that grout anchors perform best when the annulus area between the cable and the surrounding borehole wall is small. For this reason it is advantageous to provide a small diameter hole resulting in as thin an annulus as possible. Examples in the art include Scott U.S. Pat. No. 4,378,180 of Mar. 29, 1983 or Hipkins et al U.S. Pat. No. 4,477,209 of Oct. 16, 1984, or Calandra U.S. Pat. No. 4,518,252 of May 21, 1985.
A problem in using small diameter boreholes is that the cable may, even so, be quite a bit smaller in diameter than the borehole. In the process of insertion of the grout material which is normally contained within a sausage-like plastic or membrane, the cable may slip past the grout cartridge and not push it to a position for anchoring in the back of the borehole. Also, the cable may pierce or cut the grout package prematurely causing it to break and not get into proper position at the back of the borehole. Another problem is the grout escaping down the borehole and not staying in place to harden and form an anchor.
A problem in using resinous grout cable anchors is that if the grout is very fluid it can escape from the desired anchor area by flowing down the borehole in the annular space surrounding the cable. The larger the borehole, the more chance the grout loss has to happen.
Another problem is quality control of the anchor which is affected by over mixing of the resinous grout, particularly if a quick setting grout is used, say if using a 15 to 30 second formulation. To prevent over mixing it is desireable to use a more fluid grout with a gel time of perhaps two minutes. Also in large boreholes bird cages or other mixing systems must be activated for a longer period of time to assure total mixing of the resinous grout.
In certain roof support fixtures the installation effort is directed to overcoming the problems of placing long bolts in low seams which require coupling of the parts of the fixture and a weakening of the fixture due to the couplings. Couplings are known from U.S. Pat. No. 4,477,209 to increase the cost and the thread on the cable produces stress concentrations. Boreholes into which these types of roof fixtures are placed are larger, so as to accommodate the insertion of oversize couplings. When using a cable type roof bolt, for example a seven-strand cable, which may be a 5/8 inch diameter or 1/2 inch diameter cable, one can readily be placed in a borehole which may have a diameter that approaches one inch in a low seam by bending the cable in order to obtain insertion, thus eliminating the use of couplings. In any case the cable must push the grout cartridge to the back of the hole.